It is rare for a farmer to appreciate the predators that eat the animals he raises. But Miguel Medialdea is hardly an ordinary farmer. Looking out on to the carpet of flamingos that covers one of the lagoons that make up Veta la Palma, the fish farm in southern Spain where he is biologist, Medialdea shrugs. "They take about 20% of our annuel yield," he says, pointing at a blush-colored bird as it scoops up a sea bass. "But that just shows the whole system is working."

Working, indeed. Located on an island in the Guadalquivir river, 10...